Color cathode ray tubes utilized for picture image reproduction, such as in television applications and similar display media, conventionally utilize electron gun structures which provide a plurality of related electron beams to produce the desired display imagery. In operation, the modulated electron beams are scanned across the screen of the tube to provide electron impingement upon selected color-emitting phosphor areas comprising the patterned screen disposed on the inner surface of the viewing panel of the tube, whereupon the transmitted color display is reproduced. Color picture tubes of this type usually employ a multi-opening grid, mask, or other type of negative structure which is interposed between the electron gun assembly and the screen structure to provide either masking of the screen, or deflection or focusing of the electron beams thereat.
In a conventional color cathode ray tube, the electron responsive screen is usually comprised of a repetitive pattern of multitudinous dot, bar or stripe formations of various phosphor materials capable of emitting, for example, green, red and blue color luminescence upon electron beam impingement. In certain tube constructions, the pattern of phosphors comprising the screen is disposed to overlay an array of multitudinous window areas of an opaque screen matrix formed on the viewing panel. The shapings of the matrix windows and the associated color phosphor patterns constituting this type of screen structure are formed in accordance with the number of electron beams utilized and with the discrete mask openings and operative characteristics of the mask or grid structure employed in the respective tube.
Since a vast number of color phosphor groups are required to produce a high resolution display of desired color purity, the process employed in forming both the basic window matrix, and the associated phosphor screen, must be one that is capable of accurately forming a multitude of similar pattern elements in the screen arrangement. In a preferred method for fabricating the screen structure of a color tube, a photo printing technique is utilized wherein the viewing panel of the tube, having an interior coating of a photosensitive resist composition disposed thereon, is suitably photoexposed through a related negative or multi-opening mask by radiant energy means incorporated into the exposure system. Subsequent development of the numerous selectively exposed areas on the panel, produces either a first window pattern of the matrix, or in proper sequence, the associated superimposed first color phosphor pattern of the subsequently formed screen. Such photo exposure is repeated in the proper steps to form the remaining windows in the matrix and likewise to later effect deposition of the respective color-emitting phosphors associated therewith to complete fabrications of the patterned screen.
Of particular interest in this instance, is the fabrication of a tri-color screen associated with a color tube wherein the three electron guns are positioned in substantially in-line horizontal orientation, i.e., the center gun being substantially on the central axis of the tube with a remaining gun offset on either side thereof. A conventional screen for the in-line tube is one having a pattern of a multiplicity of vertical phosphor stripes, either with or without a separation matrix of narrow spaced apart opaque lines.
Associated with the vertically striped screen, disposed on the curvature of the panel, is a spatially related negative member such as a multi-opening slotted mask in the form of a slightly domed structure having a multitude of elongated openings or windows, arranged in vertical columns separated by a sufficient interstitial webbing of mask material to provide structural rigidity to the member. Thus, in this type of mask structure each vertical column of openings evidences a number of structurally separating horizontal bridge elements in addition to the integrally inherent vertical column separation material.
In separately producing either the windowed matrix pattern or the related superimposed array of electron responsive phosphor stripes, the photo-exposure system usually comprises a discrete light source to provide the actinic radiant energy necessary for the photodeposition procedure and an associated light permeable refractive medium. For the deposition of one pattern of stripes, the exposure system is positioned to substantially coincide with the central axis of the screen or panel.
The two remaining stripe patterns are achieved by shifting the exposure system horizontally to either side of the central axis. By this procedure, the resultant multi-striped screen elements are oriented to be impinged by the proper electron beams subsequently emanating from the horizontally positioned electron guns in the tube.
To photo-expose a screen pattern of continuous vertical stripes of the respective color emitting phosphor materials, by using the aforedescribed slotted mask having the horizontal bridge elements between openings, it has been found beneficial to utilize an elongated source of radiant energy positioned in a manner to be substantially parallel with the vertical columns of elongated openings in the mask. One common source of exposure radiation has been a tube-like mercury vapor lamp evidencing an incandescent arc of substantially elliposidal or elongated shaping. Such illumination, in conjunction with the light permeable refractive medium in the system, radiates a flood of actinic exposure radiation over the whole of the multi-opening mask. Moving or oscillating the light source in a linear path substantially parallel to the vertical columns of mask openings effectively increases the longitudinal source of illumination, thereby supplying exposure radiation around and under the mask bridgings. Thus, by this oscillatory motion of the light source, the shadowing of the light sensitive screen, by the horizontal interspatial bridging elements, is substantially eliminated and a screen pattern of vertical continuous stripes results.
While the oscillation of the source of exposure radiant energy is effectively realized by a motorized cam arrangement, it has been found that the amount and rate of reciprocating motion is not uniform for the fabrication of screens on the various sizes of tube viewing panels. The area of exposure, the shapings of the mask openings and interstitial webbing, the mask to panel spacing, mask and panel curvatures and other exposure considerations are all important factors to be considered in determining the amount and rate of oscillation of the exposure light source. In a properly exposed screen, the stripe patterns should be free of bridge element shadowings manifest as necked-down areas in the stripes. Thus, it was found necessary to make numerous time-consuming cam changes to finally obtain the proper set-up oscillatory stroke length for achieving correct exposure for the respective viewing panels of differing tube sizes.